How can I find out if a student's thesis is just the translation from his native language (spanish) to english?

One of my student's MA thesis is very well done. I know him and I worked with him enough to notice that something is going on. Is it possible that he just translated and paraphrased another thesis from his native language that is spanish? How can I find it out? there are citations and everthing but it's too accurate and I have some doubts.